This invention generally relates to photolithographically patterned spring structures for use in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors.
Photolithographically patterned spring structures have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring structure includes a spring metal finger having an anchor portion secured to a substrate, and a free portion initially formed on a pad of release material. The spring metal finger is fabricated such that its lower portions have a higher internal tensile stress than its upper portions, thereby producing an internal stress gradient that causes the spring metal finger to bend away from the substrate when the release material is etched. The internal stress gradient is produced by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters.
FIGS. 1(A) and 1(B) show a simplified conventional spring structure 100 consistent with that disclosed in U.S. Pat. No. 3,842,189 (Southgate). Referring to FIG. 1(A), spring structure 100 is produced by patterning a release material 120 on a substrate 110, and then forming a spring metal finger 130 that has an anchor portion 132 contacting substrate 110 and a free portion 135 extending over release material 120. FIG. 1(B) shows spring structure 100 when release material is removed and spring metal finger 130 bends away from substrate 110.
A problem associated with spring structure 100 is that formation of spring metal finger 120 over the step structure formed by release material 120 on substrate 100 produces a knee 137 in anchor portion 135. Knee 137 locally weakens the mechanical structure, thereby increasing the likelihood of breakage.
More recently developed spring structures avoid the problems described above by providing a continuous and planar release layer upon which both the free portion and the anchor portion of a spring metal finger is formed. This arrangement reduces the tendency to form a knee at the anchor portion of the spring metal finger. The contact pads, if composed of a conductive material, also serve to provide a conductive connection between the spring metal fingers and other circuitry formed on a substrate.
FIGS. 2(A) and 2(B) show a simplified spring structure 100 that is disclosed in U.S. Pat. No. 5,613,861 (Smith). Referring to FIG. 2(A), spring structure 200 is produced by patterning a release material 220 on a substrate 210 along with a metal contact pad 215, and then forming a spring metal finger 230 that has an anchor portion 232 formed on contact pad 215, and a free portion 235 extending over release material 220. Subsequently, as indicated in FIG. 2(B), release material 220 is removed, and free portion 235 of spring metal finger 230 bends away from substrate 210.
A problem with conventional spring structure 200, described above, is that separate masking steps are required to form release material 220 and spring metal finger 230. Specifically, because anchor portion 232 of spring metal finger 230 is connected to contact pad 215, a release material 220 must be masked and etched to provide an opening through which this connection is made. However, this separate masking step is costlyxe2x80x94both for mask generation, and for additional process lithography.
Another recently developed spring structure is disclosed in U.S. Pat. No. 5,665,648 (Little). Little discloses a spring structure similar to that shown in FIGS. 2(A) and 2(B), but teaches the use of a conductive (i.e., TiW) release material, instead of the non-conductive (e.g., SiN) material taught in Smith. One problem with the structure disclosed by Little is that, similar to Smith, separate masking steps are required for the release material and the spring metal finger.
What is needed is a method for fabricating spring structures that avoids the formation of knees (see discussion above directed to Southgate) and minimizes the spacing required between adjacent spring metal fingers, thereby maximizing the width and contact force of each spring metal finger after bending. What is also needed is a method for fabricating spring structures that minimizes fabrication costs by eliminating one or more masks.
The present invention is directed to efficient methods for fabricating spring structures, and the spring structures formed by the methods.
In accordance with first, second, and third disclosed embodiments, the fabrication of a spring structure includes sequentially forming a release material layer and a spring metal layer on a substrate, and then etching portions of these layers to form a spring metal island positioned on a pad of the release material. The spring metal layer is fabricated using known techniques to include internal stress variants that are later used to form a spring metal finger. According to an aspect of the present invention, both the spring metal layer and the release material layer are etched using a single mask (e.g., photoresist or plated metal), thereby reducing fabrication costs by avoiding the separate masking steps utilized in conventional methods. Etching the release layer in this manner is tricky, however, because the release material is designed to release the spring metal from the substrate when removed. According to another aspect of the present invention, premature release of the spring metal finger is prevented by etching the release material such that the release material pad is self-aligned to the spring metal island. After the self-aligned release material pad is formed under each spring metal finger, the release material is removed from beneath a free end of the spring metal island using a release mask is formed over the anchor portion of the spring metal island, leaving a free end of each spring metal island exposed through a release window. With the release material removed from beneath the free end, the internal stress variants cause the spring metal finger to bend relative to the substrate.
According to another aspect of the present invention, the spring metal finger is either etched using the same etching process used to self-align the release material, or etched using a separate etching process. In one embodiment, both the spring metal layer and the release material layer are anisotropically dry etched. In an alternative embodiment, a separate wet etching process is performed to etch the spring metal layer before an isotropic dry etching process is used to form the self-aligned release material pads.
In accordance with a fourth disclosed embodiment of the present invention, self-alignment of the release material pad to the spring metal island is achieved using a lift-off fabrication process during which both the release material pad to the spring metal island are deposited through an opening formed in a single mask. Portions of spring metal and release material deposited on the mask are then removed along with the mask, thereby leaving the spring metal island and the release material pad. Subsequent processing is essentially the same as that used in the first through third embodiments.
Spring structures fabricated in accordance with the various disclosed methods include spring metal fingers that are secured by portions of the release material to an underlying substrate. Specifically, an anchor portion of each spring metal finger is entirely formed on a portion of release material that is self-aligned to the anchor portion, so the release materials utilized in accordance with the present invention serve both the conventional function of releasing a free end of the spring metal finger, and also serve to secure the anchor portion of the spring metal finger to the substrate. As mentioned above, in order to eliminate a separate release layer masking step, the spring metal layer is formed directly on the release material layer, so the anchor portion of the spring metal finger is necessarily entirely formed on a portion of the release material. After release, that is protected by the release mask during the release operation.
Release materials used in accordance with the present invention are either conductive, or non-conductive (electrically insulating).
In addition to their release and adherence functions, conductive release layer materials (e.g., one or more of Ti, Cu, Al, Ni, Zr, Co, or heavily doped Si) further serves to conduct electric signals between the spring metal finger and contact pads formed on the substrate directly beneath the anchor portion of the spring metal finger. A portion of the conductive release material adhering the anchor portion of the spring metal finger is formed directly over the contact pad or metal via, and the adhering portion of release material acts as an electrical conduit between the contact pad/via and the spring metal finger. Note that self-alignment of the conductive release material (particularly under the anchor portion of the spring metal finger) substantially reduces the spacing required between adjacent spring metal fingers in comparison to conventional methods using separate release material masking steps, thereby facilitating spring metal fingers that are wider and, hence, stronger, and capable of applying more force.
When the release material is non-conductive (e.g., silicon nitride), the anchor portion of the spring metal finger is located adjacent to, but not over, the contact pad or metal via. In addition, the spring structure further includes a metal strap extending from the contact pad to the anchor portion of the spring metal finger. The metal strap serves both to provide electrical connection to the contact pad, and to further anchor the spring metal finger to the substrate.
The various aspects of the present invention eliminate the need for a separate masking step to pattern the release layer. The present invention also reduces the need for additional substrate metallization to lower the trace resistance in probe card and other applications of the spring structures. Further, by providing an efficient method for using conductive release materials that have substantially greater adherence to the anchor portion of each spring structure, such as titanium, the present invention facilitates a critical 3xc3x97 increase in spring thickness, and hence a roughly 9xc3x97 increase in contact force.